Main field and secular variation

摘要

Research on the main geomagnetic feld and its secular variation advanced slowly over the period 1975–1978, owing mainly to a significant drop in national support. While some achievements can be reported, we must also acknowledge the impact of this decline on the progress in new scientific results and its effect in degrading the quality of the main field models used in other geophysical disciplines.The World Magnetic Survey (WMS) (Zmuda, 1971) and work reported in the last Quadrennial Review (Cain, 1975) gave us some new insights into the main field not previously available. We obtained a rough outline of that part of the field describable by spherical harmonic wave numbers from n‐8 to about 20 and for the first time had a picture of the global anomaly structure covering the range from about 2000 Km (n = 20 = 40,000/2000) down to perhaps 400 km. However, the WMS momentum was largely lost during the last quadrennium, and these new features were not investigated in detail. As will be seen subsequently, plans for the next few years may brighten the picture considera